Why are some elements chemically inert?

Nitroeng is chemically inert. Why?

Why are d-block elements called transition elements ?

Why do some elements be monoatomic?

(A) Helium is used in modem diving apparatus (R) Helium is chemically inert

Why are the elements of zero group inert?

Why are zero group elements called noble gases or inert gases?

Why do some elements exist as molecules and some as atoms?

N_(2), CO, CN^(-) and NO^(+) are isoelectronic but the former is chemically inert and later three are very reactive, why?